Method of forming low-reflecting surfaces on optical elements



Feb. 15, 1949. m 2,461,840

.HETHOD OF FORMING LOW-REFLECTING SURFACES ON OPTICAL ELEMENTS FiledOct. 14, 1942 2 Sheets-Sheet 1 WHX I LIN/1V4 18mm or fiedericiz 11!.[Viva Li (Iflomcg Feb. 15, 1949. F. H. NICOLL METHOD OF FORMINGLOW-REFLECTING SURFACES 0N OPTICAL ELEMENTS 2 Sheets-Sheet 2 Filed Oct.14, 1942 BETH TEMPEEHTURE 'c $56? a Freda 'cfi'ii M'coll c STHLL/Nl' sa1 I z 4 ll-0 z 4 6810 DEPTH OF TPHY IN INCHES zixzst in 18W 2 GllornegPatented Feb. 15, 1949 UNITED STATES P METHOD FORMING LOW-REFLEQTINGSURFACES ON OPTICAL ELEMENTS Frederick H. Nicoll, Princeton, N. 3.,assignor to Radio Corporation of America, a corporation 1 of DelawareApplication October 14, 1942, Serial No. 461,958

I 5 Claims. (01. 41-427) and the thickness and refractive index desired:

soaps of fatty acids (Blodgett Patents Nos.

2,220,860 and 2,220,861) or fluorides (Cartwright for minimizingreflection and promoting transmission of light, and to low reflectionglass produced by such treatment.

This application is a-continuation-in-part of my application Serial No.406,601 filed August 13, 1941, now abandoned.

The optical characteristics required for a film on glass to secureminimum reflection and maximum transmission of light are well known. Asto thickness, the film should have an optical thickness of one-quarterof the wave length of the light to be transmitted. In the case of whitelight, the thickness usually desired is one-quarter of the wave lengthof the green component, which is approximately 5350 angstroms. In lieuof one-quarter wave length, the thickness may be an odd number ofquarter wave lengths, but with loss of efilciency as, the odd numberincreases. Any deviation in the thickness, of the film from a quarterwave length, or an odd number of quarter wave lengths, also reduces theefilciency of the film. As to the index of refraction, the film shouldhave a lower index than that of the glass and preferably it should be ofthe order of the square root of the product of the indices of refractionof the glass and the other medium between which the film is to be used.If the other medium is air, the ideal index of refraction of the filmwould be approximately the square root of that of the glass. However,useful suppression of reflection may be obtained by imparting to thefilm a refractive index substantially.

use. To obtain a desirably low index of refraction for the film 'it hasheretofore been necessary to resort to coating the glass with softer andless et al. Patent No. 2,207,656).

Reflection reducing films have also been obtained by leaching certaintypes of glass, such as lead glass, with aqueous solutions of reagents,

such as nitric acid, which will not disolve silica 5 but are capable ofattacking-and dissolving certain non-silicious constituents of theglass, insofar as such non-silicious constituents are not blocked offfrom such attack by silica or silicious reaction products (OrdnanceDepartment Document No. 2037, May 1921, page '76, and Blodgett Patent2,220,862 Such leaching processes, being dependent on, the compositionof the particular glass, give irregular results. Moreover, the index ofrefraction has only been reduced to approximately 1.46, the index ofsolid silica (fused quartz) (Blodgett articles, The Physical Review forFebruary 15, 1939 and The Review of Scientific Instruments for January1941) The limited and inadequate reduction of'the refractive index thathas been obtained by leaching glass with solutions of such reagents,shows that no substantial skeletonization of the film was obtained bysuch leaching process. The substantial lack of skeletonized structure insuch leached films is recognized in the art (Blodgett article, ThePhysical Review for Feb. 15, 1939, at page 402).

It is known that leaching glass 'in a hydrofiuoric acid solution doesnot change the character of the surface of the glass to produce a lowrefiection film, as does leaching lead glass in a r nitric acidsolution. The reaction of the hydrofiuoric acid solution depends uponits concentration and the extent to which it is agitated. Concentratedsolutions tend to pit the glass and fgive it a frosted appearancewhereas dilute solutions remove the glass more slowly but substantiallyuniformly; leaving a polished surface having substantially the sameindex of refraction as that of the original surface. Agitation alsotends to reduce the pitting and produce a more unifprm removal of theglass. In an experiment with glass partially immersed in a dilutesolution of hydrofluoric acid to further study this known reaction, Idiscovered that an entirely different reaction was taking place along a,very narrow resistant substances, such as stearates or other -surfaceof the solution and that at such narrow ,lead glass containing as muchas 66% well as to light and intermediate lead glasses,

band the surface produced on the glass by this reaction had low.reflection properties. I found that I could produce a similar lowreflection surface with substantial uniformity over the entiresurface ofa piece of glass by treating the glass with vapor obtained from a dilutesolution of hydrofluoric acid and controlled as herein explained. I alsofound that the low reflection surface produced by this vapor reactionmay be made to have a thickness of any desired quarter wave length ofvisible light, and a hardness, durability and resistance to chemicalreaction comparable to that of glass itself in combination with arefractive index lower than was previously obtainable except by coatingthe glass with softer materials.-

Broadly stated, my method comprises treating the surface of glass withdilute hydrofluoric acid gas under conditions promoting substantiallyuniform gas concentration and distribution and preventing condensationof moisture, at the treated surface, until a region extendinginwardlyfrom the surface, integral with the glass, skeletonized to the desiredextent and having the desired thickness is formed. While thisskeletonized region is herein called a layer, it is not to be confusedwith the physically deposited layer added to the surface of glass byearlier methods.

My process attacks silicious as well as non- -silicious constituents ofglass, and may be used to skeletonize a region extending inwardly from vthe surface of any kind of glass having silicious and non-siliciousconstituents susceptible to attack by hydrofluoric acid. For example, myprocess has been successfully applied to the treatment of window glassand other lime glasses, to lead. as

to borosilicate crown glass containing as much as 70% silica, as well asto such glass of relatively low silica content, and to barium glasses ofvarious types.

A convenient apparatus for carrying out my process and a series ofcurves showing operating characteristics for such apparatus, areillustrated in the accompanying drawings in which:

Fig. 1 is a sectional view of the apparatus, and

Figs. 2 to5 inclusive are graphs representing the relation between time,temperature, concentration of solution and other factors when theprocess is carried out with apparatus similar to that illustrated inFig. 1.

Before treatin the glass it is usually desirable to clean it thoroughly.Finger marks and grease should be removed by washing the glass with asolvent such as alcohol or acetone. The surface of the glass can then begiven a thorough cleaning with powdered chalk or one of the equivalentcommercial cleaning preparations and the sur- -face thoroughly wipedbefore the powder dries The surfaceshould finally be polished with a dryI cloth until-no marks are visible.

- The glass can be treated by means of the tray apparatus illustrated inFig. 1, which comprises 65 data for Fig. 2, a tray depth-of two incheswas a tray-like container I lined with paraflln, wax or other similarmaterial II for resisting attack by hydrofluoric acid. The tray ispartly fllled with a dilute aqueous solution of hydrofluoric 'acid asindicated at I2, this being a convenient way to provide a controllablesource of dilute hydrofluoric acid gas for use in the process. The glassto be treated, indicated at I3, is placed across the top of tray I0whereby its lower surface I4 is attacked by vapor from the solution 12.Other controllable sources of hydrofluoric 60 reflection layer.

5 tion for carrying out the process at ordinary room temperature.

Weaker solutions slow the process unnecessarily and stronger solutions,especially solutions exceeding 5%, make it more dimcult to avoidundesirable etching of the glass.

However, the strength of the solution may be varied as hereinafterindicated, with conditions 1 which affect the concentration of the gasat the surface of the glass, since it is the latter concentration thatis of importance in carrying out the process.

Container I0, is placed in a larger container I5 and spaced from thebottom thereof by spacers I8 so thats water bath Il may be used tocontrol the temperature of the hydrofluoric acid solution I2. =When theglass I3 is at room temperature satisfactory results are obtained if thebath temperature is from 5 C. to 7 0. lower than room temperature. Ifthe bath temperature approaches the glass temperature too closely, thehydrofluoric acid vapor condenses on the glass and either damages orremoves the low reflection surface.

It is usually desirable to maintain an air-tight contact between theglass and the tray-during the treatment so as to avoid non-uniformity in3 the low reflection surface near the edge of the glass. This canbeaccomplished by use of sealing tape. sponge rubber strips, soft wax orother similar sealing materials.

The treatment is continued until the treated 5 surface, .which can beexamined from time to time without removing the glass from the tray.produces a purple interference color-which is visible in reflectedlight. Under this condition maximum transmission of green light occursand reflection from the treated surface is usually between .2% and .4%of the incident light.

During the treatment a white deposit consisting of fluorides and otherproducts of the chemical reaction, is formed on the treated surface.

The deposit should be removed by washing the glass in water as soon asit is removed from the tray. If allowed to remain on the glass, thedeposit becomes very'hard and causes the treated surface to have ahigher index of refraction than if the glass were promptly washed inwater.

When my process is carried out by means of the tray method there arecertain variables which determine the satisfactory operation of theprocess. The relation of these variables to each other is shown by thegraphs of Figs. 2 to 5.

Fig. 2 shows the time in hours required to treat ordinary crown glass,for difl'erent concentrations of the hydrofluoric acid solution anddifferent bath temperatures, to produce the desired low These variablesare also dependent upon the tray depth (the distance between the lowersurface I4 ofthe glass and the solution I2) and the temperaturedifference between the bath and theglass. In obtaining the used and thetemperature difference was between 5 C. and 7 C The ordinates are givenin hours required to reach the 1st minimum, 1. e., the

condition when the skeletonized layer has a 7 thickness, or equivalentoptical thickness, equal drofluoric acid is increased and also as thebath temperature is increased. Thus the process follows the usualchemical law relating to the increasing rate of chemical reaction withincrease in temperature.

Satisfactory operating conditions for carrying 'out the process can bedetermined from Fig. 2. For example, with a tray depth of two inches anda hydrofluoric acid solution of 1.% concentration, the time of treatmentwill be Just over six hours if the bath temperature is C. and the glassf temperature is suiflciently higher to produce a satisfactory lowreflection layer in: accordance with the graphs shown in Fig. 5. If theglass tem- 'perature is from 5 C. to 7 C. higher than the bathtemperature, very satisfactory results are obtained. Increasing the bathand glass temperatures while maintaining the desired 5 C. to

7 C. temperature diflerence, will decrease the time of treatment. If thebath temperature is increased to C., whi'e maintaining the desiredtemperature difference, the time of treatment is 'reduced to about fivehours. Increasing the concentration also decreases the time oftreatment.

as shown by the 2% concentrationline.

Fig. 3 shows in more detail how the time of -treatment varies withvariations in the concentration of the hydrofluoric acid solution. The

working conditions for Fig. 3 also include a tray depth of two inchesand a temperature difference within the range indicated in Fig. 5 andpreferably between 5 C. and 7 C. The graph shows that the time oftreatment decreases as the uniform low reflection layer when treatedwith a tray depth or eight inches and an acid concentration of 4%.

Fig. 5 shows conditions under which satisfactory low reflection layersare obtainable. With a tray depth of two inches satisfactory layers areobtainable for conditions falling within the area under the curve markedTray 2" Deep." Thus, with a bath temperature of 17 C. and aconcentration of 1%, satisfactory layers are obtain-,

- indicated as satisfactory on the graph. 0n the other hand, the softerlayers usually have lower indices of refraction due, probably, to moreextensive skeletonization of the layers. If the temperature differencefalls below ill 2 C. the surface becomes quite soft as indicated by thelegend at the right hand side of the graph. If the temperature of theglass is increased so that the temperature difference is somewhatgreater than 8 0., the layer usually becomes crystalline 'as isindicated by the legend at the left hand .side of the graph.

concentration of the hydrofluoric acid solution is -'increased. With abath temperature of 17C. and with 1% concentration, the time oftreatment is over seven hours. is increased to 2% the time is reduced toabout,

If the concentration four and one-half hours whereas if the concen-'tration isincreased to 4% the time is reduced to less than three hours.If the concentration is increased above 4% and the other variables aremaintained constant; the time of treatment will be correspondingly.decreased but the surface will Increasing the concentration from 1% to2% decreases the operating limits within which satisfactory lowreflection layers are obtainable.

At 2% concentration satisfactory layers are obtainable if thetemperature difference between the bath and glass, is between 2 C. and 7C.

probably be soft and crystalline as shown by Fig. 5. Fig. 3 also showsthat if the bath temperature is increased there is a correspondindecrease in time of treatment.

Fig. 4 shows how the tim of treatment varies with thetray depth." As thetray depth is increased the time of treatment is likewise increased. Thedata for Fig. 4 was obtained with -the bath temperature maintained at 17C. and

the glass temperature sufllciently higher to give satisfactory lowreflection layers, e. g., to maintain the desired 5 C. to-7 C.temperature difference. With a tray depth of two inches and 1%concentration, the time of treatment is approximately seven hourswhereas if the tray depth is reduced to one inch the time of treatmentis decreased to about four and one-half hours.

Under these conditions the layer is harder if this temperaturedifference, is approximately 6 C. or If the concentration is increasedto 3% sat sfactory layers are obtainable for temperature differencesbetween 3 C. and 6 C. whereas if the concentration is increased to 4%satisfactory layers are obtainable when the temperature difference is 4C. If the concentration is increased above 4%; the layer tends to becomesoft and crystalline as is indicated by the legend appearing at the topof the graph.

Similarly, the conditions under which suitable low reflecting layers canbe obtained for tray "depths of one inch and of four inches can be."determined from the curves labeled Tray 1" Deep" and "Tray 4" Deep.

- The tendency to form a crystalline pattern re- 'sulting from too greata temperature difierence the tray depth is increased to four inches thetime Y of treatment is increased to over ten hours. However, if theconcentration is increased at' the same time that the tray depth isincreased,-the

time of treatment can be maintained substantially constant. 4

- In treating curved surfaces it is desirable to increase the traydepthand concentration as there is substantially less variation between dif-I ferent areas of the treated curved surface for La tray depth of, say,eight inches than for a tray depth of one inch. Treatment of a watchcrystal approximately three inches indiameter and approximatelyone-quarter inch higher at the center than at the edge; produced asubstantially terial on the glass. efiective cleaner. In fact, mostacids and alkalies between the glass and the bath, is decreased byremoving the glass from the tray from time to 'time and washing it withwater. By'this means the temperature may be increased a few degrees andthe time of treatment correspondingly decreased. This procedure has thedisadvantage,

'however. that the final result is determined largely by inspection asthe interruption of the treatment and the consequent variations intemperature preclude treatment controlled as a function of time andtemperature.

After the glass has been treated it can be cleaned in any manner inwhich ordinary glass is "cleaned except that the use of abrasives shouldbe avoided. The skeletonlzed layer can, for instance, be cleaned withliquid window cleaners or with a solvent for the specific foreign ma-Soap and water is a very largely of silicious nature such as the factthat it is not appreciably attacked by many reagents which readilyattack calcium fluoride and other fluorides, as well as thenon-silicious ingredients of the glass. Among such reagents are ammoniumcarbonate solutions, dilute and concentrated I-nsoi, cold dilute HNOa,cold NAOH, concentrated HCl and chromic acid either cold or boiling.

. the order of the square root of the index of re- Calculations fromreflection measurements obtained from one sample of treated glass showthat the low reflection layer of that sample has an index of refractionin the neighborhood of 1.28 as compared with 1.46, the index ofrefraction of solid silica. The indices of refraction of other lowreflection layers produced by my process are likewise appreciably lessthan 1.46 although the index of refraction of any particular layer maydiffer from that of another layer depending upon the nature of the glasson which it is formed and other factors such as the temperatures atwhich the process was carried out and the concentration of-thehydrofluoric acid solution. From these facts it is concluded that thelow reflection layer is porous rather than a fllm of solid silica, andthat the reduction in the index or refraction to a value appreciablyless than the refractive index of solid silica results from thereduction in the density of the glass (or silica) by the formation 'ofthe skeletonized or coral-like structure which constitutes the lowreflection layer. An index of refraction of 1.28 for a material composedlargely of silica, corresponds to the refractive index of a structurehaving a solid content of the order of 65% and minute air-filled voidsof the order of 35%.

Another indication that the low reflection layer produced by my processhas a skeletonized structure, is the fact that light oil applied to itis absorbed into its structure in such a manner that the oil cannot beremoved by wiping the treated surface with a dry cloth. It is necessaryto wash the glass with soap and water or to use some other solvent toremove the oil. Finger prints cannot be wiped from the treated surfacebut must be I verse dimensions in the neighborhood of onefortieth of thewave length of green light and that the distances between voids,measured through the solid portions of the skeletonized structure, arelikewise substantially smaller than a wave length of visible light.

I'claim:

1. The method of treating glass to reduce the reflection of a componentof light therefrom which comprises the steps of exposing a surface ofsaid glass to the action of vapor arising from a dilute aqueous solutionof hydrofluoric acid, maintaining the temperature of said solution lowerthan that of said surface to prevent condensation. whereby to reduce thedensity of a layer of said fraction of said glass.

2. A method according to claim 1 wherein said surface is maintained freefrom condensation by maintaining said solution at a temperature ofbetween 16 C. to 27 C. and said surface is maintained at a temperatureof from 5 C. to 7 0. higher than that of said solution.

3. The method of reducing the index of refraction of a region of uniformdepth extending inwardly from a surface of glass which includes the stepof subjecting a surface of said glass to hydrofluoric acid vapor equalto that existing at a point two inches above an aqueous solution ofhydrofluoric acid of one percent concentration under conditionsinhibiting condensation of said vapor on said glass by maintaining saidgas at a lower temperature than said glass to skeietonize the surface ofsaid glass by removing silicious and non-silicious components of saidglass until a porous structure consisting substantially of silicaremains, but having an index of refraction substantially lower than thatof solid silica, and an optical thickness equal to an odd nuinber ofquarter wavelengths of a component of visible light.-

4. The method of treating a glass surface to I reduce the reflectiontherefrom of light of a given wavelength which consists in altering saidsurface .ber of quarter wavelengths of said light.

5. The method of treating glass which includes the steps of subjecting asurface of the glass toan aqueous hydrofluoric acid vapor having aconcentration of water vapor equal to that at a point two inches abovean aqueous solution of hydrofluoric acid of one percent concentration,maintaining the temperature of the glass at approximately 23 C. and thetemperature of the solution from 5 to 7 lower, and continuing suchtreatment until a low reflectance surface layer is formed having athickness equal to an odd number of quarter wavelengths of a componentof visible light.

FREDERICK H. NICOLL.

REFERENCES CITED The following references are of record in the flle ofthis patent:

UNITED STATES PATENTS (Other references on following page) Numberflumber UNITED STATES PATENTS Name Date Yunck May 28, 1940 Cartwright etal July 9, 1940 Hood Sept. 17, 1940 'Blodgett Nov. 5, 1940 Rayton May12, 1942 Adams May 16, 1944 Weissenberg Aug. 22, 1944 Wolfskill Dec. 5,1944' FOREIGN PATENTS I Country Date Great Britain 1904 OTHER REFERENCESPottery and Glassware, Dec. 1, 1887, Etching on Glass. 5 Jones et a1.J.O .S.A., vol. 31, Jan. 1941, pp. 34-

Blodgett, Physical Review, vol. 55, Feb. 15, 1939, pp. 402, 403.

Deve: Text Optical Workshop Principles, 1942- 10 1943, Translated fromFrench edition (1936) by Trippel, pages 279-281, Supplied by Jarrell-Ash(20., 165 Newbury Street, Boston. Mass;

